1. Field of the Invention
The present invention relates generally to luminescent glasses, and more specifically to glasses doped with nanocrystalline semiconductor particles and an activator for luminescence.
2. Description of the Background Art
The luminescence of bulk semiconductor and insulator materials (herein generally referred to as inorganic solids) has been intensively studied for decades. The nature of the luminescent centers in these materials can be broadly classified into several categories, 1) emission due to recombination of electron-hole pairs; 2) exciton emission; 3) broadband emission due to impurities with filled electronic shells; and 4) narrowband emission due to impurities with incomplete electronic shells, such as transition metal and rare earth ions. The impurity atoms and ions are referred to as activators (and/or co-activators if more than one impurity is required). Luminescent inorganic solids, particularly those activated by added impurities, are known as phosphors. There are countless important commercial and military applications for a wide variety of phosphor materials. A brief (and necessarily incomplete) list of phosphor applications follows:
1) Cathodoluminescence
Cathodoluminescent phosphors are used in cathode ray tubes (CRT's), including television sets, radar screens and oscilloscope displays. Typical cathodoluminescent phosphors are sulfides of cadmium and zinc. The colors emitted by a color television screen are due to the interaction of accelerated electrons with phosphors activated with impurities selected for the frequency (color) of their luminescence.
2) Electroluminescence
Electroluminescence is the generation of light upon the application of an electric field across a material. The applications for electroluminescent phosphors are numerous, including, for example, lighting and display technologies.
3) Thermoluminescence
Thermoluminescent phosphors emit light when heated. Emission occurs due to the release (detrapping) of trapped electrons that result from prior excitation of the phosphor using ultraviolet (UV) or ionizing radiation. Thermoluminescent materials are used as dosimeters to monitor the exposure of personnel and equipment to high energy ionizing radiation.
4) Radioluminescence
Radioluminescent phosphors emit light upon exposure to high energy ionizing radiation and are often referred to as radiation scintillators. Inorganic scintillators are used to detect the presence of ionizing radiation in many venues including monitoring of the environment and the protection of personnel at nuclear installations.
5) Sensitized Luminescence
Infrared (IR) stimulable phosphors emit visible light upon exposure to infrared radiation. This emission occurs due to the migration and recombination of trapped electrons which previously formed upon excitation of the phosphor by UV radiation. These phosphors are similar in principle to the thermoluminescent phosphors in that the excitation energy of the UV radiation is stored (trapped) in the phosphor. These phosphors have many applications including, for example, the detection and imaging of IR radiation, and optical data storage.
Many of the inorganic solid phosphors used in the applications cited above are available only as polycrystalline powders with particle dimensions ranging from one to tens of microns. An intensive area of research for literally decades has been the search for improvements in the structure and form of the phosphors. Attempts to grow single crystal phosphors, thin film phosphors, or phosphors embedded in glass have met with varying degrees of success. To better illustrate the motivation for these efforts, consider a typical polycrystalline phosphor, available as a powder with particle dimensions of 1 .mu.m or greater. Large crystals often cannot be grown due to the very high melting point of the inorganic solid and the presence of the activator metal ions. The powder appears white when illuminated with visible light due to the highly efficient scattering caused by the small particles. Because of the efficient scatter, the phosphor powder is not transparent to its own luminescence. For this reason, the phosphor must be used in a very thin layer or its luminescence would be effectively attenuated. A final disadvantage of the powder phosphor is its mechanical fragility. It must be protected somehow and usually cannot withstand high temperatures, hostile chemical environments, or abrasions. Severe limitations are often placed on the functionality of the phosphor due to the inability to manufacture the material in a transparent state. The problems cited above are not unique to any one application, but are shared by many phosphors in many applications. Even in those cases where it is possible to grow the phosphor in a single crystal, many problems remain. The crystals may be mechanically fragile or susceptible to thermal shocks. Some crystals are hygroscopic, or cannot withstand even mildly corrosive chemical environments. Some phosphors become health hazards as they age due to the diffusion of toxic materials out of the crystal.
In order to avoid some of these difficulties, researchers have worked to incorporate polycrystalline phosphors into glass matrices. Historically, two basic approaches have been used. Attempts were made to grow phosphor nanocrystals from the substituent inorganic material and activators) by diffusive precipitation from a glass melt as a result of striking (heat treating) the glass. This approach did not meet with success since the activator ions are usually quite soluble in the glass matrix and prefer to remain in the glass rather than precipitate out with the crystal. The second technique has been to embed the micron-sized crystalline phosphors in a glass (or polymer) matrix, Although polycrystalline phosphors have been incorporated in this way in both low-melting glass and polymer hosts, the optical problems (scattering) associated with the micron sized particles are not improved.
B. Nanocrystalline Inorganic Solids
Nanometer-sized crystals of inorganic solids, and, in particular, semiconductors, have been intensively studied over the past decade due to the interest in the basic physical properties of the nanocrystals and their potential uses in electronic and optical devices. The effects of the small dimensions on the physical properties are often referred to as quantum confinement effects and the nanocrystals themselves are often referred to as quantum dots.
Several techniques have been developed to grow or deposit semiconductor quantum dots in glass matrices. Nanocrystals of both I-VII and II-VI semiconductors have been grown in aluminoborosilicate glasses using the diffusive precipitation method described above. II-VI semiconductor quantum dots have been prepared using a variety of sol-gel glass fabrication methods as well as radiofrequency (rf) sputtering methods. II-VI semiconductor quantum dots have been deposited into porous Vycor.TM. (Corning, Inc) glass using precipitation reactions as well as metalloorganic chemical vapor depositions (MOCVD or CVD) methods, III-V semiconductor quantum dots have also been prepared in porous Vycor.TM. glass using several CVD techniques.
Previous studies of semiconductor quantum dots have predominantly focused on the optical properties, and, in particular, the nonlinear optical properties of the quantum dots. The materials processing issues have concerned the purity (stoichiometry) of the quantum dots, the crystal size and distribution, and the crystallinity. Luminescence from semiconductor-doped glasses, such as commercially available cadmium sulfide/selenide glasses, has been measured at room temperature. The luminescence appears as a narrow feature near the band edge, attributed to direct recombination, and a broad, less intense red-shifted band attributed by various authors to shallow surface related traps or defects. The luminescent features were found to be dependent on the stoichiometry of the semiconductor. Exciton and biexciton luminescence from CuCl quantum dots has been observed at low temperatures (T.ltoreq.108 K.) and lasing due to the biexciton to exciton transition has been reported, also at 77 K.
There have been no successful prior efforts to develop activated nanocrystalline phosphors within a glass matrix, followed by appropriate heat treatment.